The present invention relates to radio frequency tags for communication, identification and distance measurement, in particular to a system of radio frequency communication with addressed unicast and multicast tags with wide-band technology.
Radio Frequency IDentification (RFID) is a method of storing and remotely retrieving data using devices called RFID tags. An RFID tag is a small object that can be attached to a product, animal, or person. RFID tags receive and respond to radio-frequency queries from an RFID reader. RFID tags can be either active or passive. Passive tags require no internal power source, whereas active tags require a power source. Active RFID tags have an internal power source, and typically have longer range and larger memories than passive tags.
An RFID system includes several components including mobile tags, tag readers, and application software. The RFID system enables a query to be received by the mobile tag and the tag responds with data. The data is received by an RFID reader and processed according to the needs of a particular application. The data transmitted by the tag may provide identification or location information, or specifics about the product tagged, such as price, color, date of purchase.
RF identification (RFID) systems are used to track objects, animals and/or people in a large range of applications. As an example, RFID is used to track books in a library. Security gates includes an RF transceiver as part of the RFID reader which detects whether or not a book has been properly checked out of the library. When the book returns, the tag attached to the book is detected and an appropriate record is updated in the library system. In another application, RFID readers previously located in a warehouse are used to identify certain objects (for example, on a track entering the warehouse), or to find the location of certain objects, by communicating with their tags and measuring the position of their tags.
In many cases it is desired to have several communication links in parallel between readers and tags or between tags to support various network topologies and support a high aggregate rate per a given space. For simplicity, RFID systems do not include time synchronization between elements of the network. In such a network of the prior art there are “collisions” between simultaneous transmissions (namely two or more transmissions using the same time, frequency or code space). The RFID system has media access control to support various networking modes including retransmission of possibly lost messages due for instance to collisions.
A method for avoiding collisions as used with the ALOHA protocol is described in “Computer Communications”, Principles and Business Applications, by Andy Sloane, Mcgraw Hill ISBN 0 07 709443 3. After a collision, the transmitter waits a pseudo-random period before re-transmitting the signal, so that the chance of a collision occurring between the retransmitted signals is small. The random periods are longer than the transmitted signals, such that response signals of transmitters selecting different random delays are not likely to collide.
In certain RFID applications, it is important to ascertain that the identified tag is located within a certain distance from the reader. For example, the identification of the tag may be required in order to open a door to an access-limited area. If a tag which is positioned remotely from the reader is identified by the reader, the door may be opened to an unauthorized individual. Limiting transmission range of the reader is not a potential solution to this problem because near the limit of the transmission range tags may not be identified for instance due to different tag orientations and/or on occlusion between the tag and reader.
In U.S. Pat. No. 6,362,738, issued to Vega, an RFID reader is disclosed containing a detector circuit for detecting the presence of a signal carrier frequency transmitted by the transponder in response to a signal from the reader. The detector circuit has a resonator circuit which is connected to a receiver electrode. The resonator includes a piezoelectric element with a high quality factor ‘Q’ at the resonant frequency to enhance sensitivity. The alarm carrier signal is rectified and fed to either a peak detector or an envelope detector circuit. A voltage source generates a voltage threshold to allow for operating range adjustment. A comparator compares both voltages and generates an alarm signal if the voltage signal reaches the threshold voltage. The system disclosed in U.S. Pat. No. 6,362,738 limits the range of the tags it identifies by relating only to signals whose voltage level is above a predetermined threshold. The threshold may be user-adjusted in order to allow for different ranges of operation. The use of power thresholds is inaccurate, as the power may depend on the orientation of the tag and/or on obstructions between the tag and the reader. Thus it would be desirable to be able to measure the distance, or relative distance of specific tag(s) from a reader, possibly using other tags whose location is known.
As stated above, a shortcoming of conventional RFID systems, is a limited ability to establish multiple parallel links, for instance between a reader and two tags. Reference is now made to FIG. 1 which illustrates schematically RFID signals between an RFID reader and a tag. Commonly, a reader transmits a packet 101, and waits for a tag to respond with a packet 103. Typically, packets 101 and 103 are on the order of 1 millisecond long. Because of relatively long packet length, the ability to achieve multiple parallel links is limited.
PCT International Patent Application Publication No. WO 2003/098528, (PCT Patent Application No. PCT/IL2003/00358), by the first inventor of the present invention, entitled “Method and system for distance determination of RF tags” is incorporated by reference for all purposes as if fully set forth herein. PCT/IL2003/00358 discloses an RFID system having the capability of automatically identifying unknown tags by sending a broadcast interrogation wide-band message signal and receiving responses from all tags that receive the message signal. Reference is now made to FIG. 2 (prior art) which illustrates the use of wide-band signals (UWB) in an RFID system 10 of the prior art (described in PCT/IL2003/00358). A reader 201 transmits a pulse sequence or symbol 205. Preferably, a wide band signal is organized into three intervals including three parts: a preamble, data and a response period. In each of these parts, symbols 205 are transmitted by means of pulse transmissions, where the time between symbols is denoted T1 typically on the order of 10 microseconds. The actual pulse sequence transmission time T2 for each symbol is substantially shorter than T1, typically ˜100 nanoseconds. Such pulses are beneficial for reducing the peak to average ratio of the transmitter, both for easier implementation and for passing regulatory peak power limits where applicable. As an example in a pulse train, each pulse sequence 205 is composed of N e.g. 11 narrow pulses, each with a polarity determined by a binary sequence which is chosen for autocorrelation and synchronization properties with a flat spectrum. Tags 203a and 203b respond respectively with pulse sequences 207a and 207b also with time interval T2 of about 100 nanoseconds and time interval T1 between pulse sequences 207 (on the order of 10 microseconds, as mentioned above). The use of very short pulse sequences 205 and 207 with a long time interval between pulse sequences 205 and 207 allows a relatively large number of parallel links between reader 201 and multiple tags 203.
However, several features useful in RFID networks are not supported using the prior art system of PCT/IL2003/00358. A prior art RFID system which uses broadcast wide band signals cannot readily communicate with a relatively distant tag 203 or a tag 203 which is situated behind an attenuating obstacle, using another addressed tag acting as a relay. Moreover, the system of PCT/IL2003/00358 is not suitable for measuring distance between a reader 205 and a tag 203 by triangulation which employs other tags 203.
There is thus a need for, and it would be highly advantageous to have a system for RFID wide band communications which overcome the above mentioned disadvantages of prior art wide band RFID Systems.
The term ‘channel’ as used herein refers to an allocation of resources providing a link between a transmitter and a receiver. Exemplary channels are frequency band, time slot, space direction and spreading code. The term “wide band signal” as used herein refers to a spread spectrum signal type such as: direct sequence (DS), frequency-hopping (FH), multi-carrier CDMA, chirp signals, short or long pulses of any shape with or without time hopping. The terms “wide-band” and “spread spectrum” are used herein interchangeably. The term “reader cell” or “cell” as used herein refers to a previously determined bounded volume, such as a volume bounded within a specified radius or between two radii, the volume determined by the ability to successfully achieve wireless communications between a reader and tag in the volume. Tags located in the reader cell are valid tags for the current session. The term “signal’ as used herein refers to one or more signals in one logical transmission period. The term “broadcast” as used herein refers to a message that are intended for any receiver of the system and consequently does not include an identifier or address of one or more intended receivers. The term “unicast” is used herein to refer to a message intended for and addressed to a single recipient. The term “multicast” is used herein to refer to a message intended for and addressed to multiple recipients. The term “addressed message signal” or “addressed wide band message signal” or “addressed wide band response signal” as used herein refers to a unicast or a multicast message signal, i.e. to a message signal that addresses one or more previously defined system elements, i.e. tags or readers by including respective identifiers in the signal. Although the term “message signal” usually refers to a query sent by a reader and the term “response signal” usually refers to a response sent by the tag to the reader in response to the query, in the context of the present invention, the terms “message signal” and “response” signal are used herein interchangeably. In particular, in the context of the present invention, when a single message may be both a query and a response, the term “response signal” is typically used. The terms “round trip delay time” “time delay”, “delay time” or “time information including time of flight” are used interchangeably and refer to an interval of time as measured by a single system element between transmitting a transmitted signal and receiving a response signal in response to the transmitted signal. Reference: http://en.wikipedia.org/wiki/RFID